In a copper dry smelting process, an imbalance is caused between the supply of raw materials such as a copper concentrate and a silica concentrate, and the supply of oxygen into a furnace. As a result, the amount of oxygen becomes too large relative to the raw materials. In this case, magnetite (Fe3O4) is generated in the slag. Since a Fe3O4 layer or a layer containing a large amount of Fe3O4 has a higher melting point than the slag existing near the layer, the layer is not put into a liquid state, but remains in a semi-molten state in the furnace. As a result, the semi-molten layer blocks a slag tap hole, and hinders the operation of the furnace by reducing the inner volume of the furnace, for example. Also, a layer containing a large amount of Fe3O4 has high viscosity, and might hinder precipitation separation of a valuable metal such as copper suspended in the slag. This might lead to a decrease in valuable metal collection rate.
Therefore, to lower production costs and increase the valuable metal collection rate, it is essential that generation of Fe3O4 is restrained in a copper dry smelting process.
Japanese Patent Application Publication No. 58-221241 discloses a technique for restraining generation of Fe3O4. By this method, pulverized coke and pulverized coal as well as a copper concentrate are blown onto the slag surface, and the Fe3O4 is reduced to FeO by the pulverized coke. In this manner, the viscosity of the slag is lowered.
Japanese Patent No. 3217675 points out the problem that the coating layer of the furnace refractory is damaged due to excessive reduction when the amount of added coal is too large, and discloses appropriate conditions such as the controlled particle size of coal, the controlled concentration of each component, and the likes, so as to achieve appropriate reactivity between Fe3O4 and pulverized coke. Japanese Patent No. 3529317 discloses a technique for reducing Fe3O4 to FeO by adding granular pig iron (metallic iron) to an intermediate layer formed between matte and slag.
A copper dry smelting process has the advantage that the oxidation heat of copper ore can be used to melt the copper ore. According to any of the techniques disclosed in the above references, however, it is necessary to use a coke material as supplemental fuel. As a result, the production costs become higher.